Imagine a situation where you have multiple custom client fields and it feels overwhelming for a new client to fill on all the data the first time they visit your website. Having abillity to postpone filling some of those data would be usefull. You could say that you have view on which you could do just that, but at the moment there is no way on forcing client to fill address, city etc. at the time of purchase.
In ideal scenario client would register with just an email and password and at the time of purchase was prompted for the rest of the details.
Both standard WHMCS fields and custom client fields should have option to be filled during checkout (similarly to unregistered client).
As for configuration i think that each of the standard and custom fields should have checkbox to determine whether field should be filled during checkout and skipped at registration time or required at first register.